Scientific Method —

Arrow of time no longer double-ended

A physicist claims to have solved one of the more persistent problems in …

Sometimes, halfway through reading a paper, I wonder if the editors have been Sokaled (if this is not a word, it really should be), fooled by a vaguely scientific-sounding parody. This is one of those occasions. Lorenzo Maccone, an itinerant physicist currently residing at MIT, has published a paper that claims to solve one of the trickier problems facing physics: why does time have a direction?

Backing up a bit—all the laws of physics are time agnostic. You can run time forward or backwards, and it makes no difference at all. Literally, as far as physics is concerned, there is no reason why we experience time in the direction we do.

The exception to this rule is entropy, which always increases in a closed system. Entropy is based on irreversible physical processes, where running time backwards doesn't get you back to where you started, despite the fact that there are no physical laws that allow this to occur. So, although we observe experimentally that entropy always increases, there are no known irreversible processes that can drive entropy.

Maccone has taken a slightly different view of this problem by looking at correlations. Imagine I do something that increases entropy slightly, and my wife observes the results of my actions and records the consequent increase in entropy—we will leave the fight over who should tidy up the mess out of the story.

Now, I can choose a set of operations that can return the entropy to its previously low value. However, doing so involves not just reversing my actions, but also reversing all correlated systems. In other words, I have to wipe my wife's memory of the event and her subsequent recording of it. If she wrote it on a piece of paper, I have to wipe the paper clean etc, etc. But at the end of it, there would be no record of the event ever having occurred.

The upshot is that entropy-decreasing events can occur, but can never be observed from within the system. You can extend this to the universe, which may well be a closed system: we are within it and, even though events that reduce the entire entropy of the universe are possible, we can never observe such things.

How does this resolve the arrow of time problem? Well, put simply, running time in one direction allows records to be kept and events to be observed. In the other direction, observation becomes impossible. Therefore, although time could be running in either direction (or, who knows, both directions simultaneously), it is only possible for any observer (not necessarily a human one) to experience time in the forward direction.

So, have we been Sokaled? Probably not. Although Maccone has an unusual employment history, he is definitely a physicist with a publishing background on related topics. Nevertheless, the pictures that appeared in my head when reading this paper were priceless.

Chris Lee
Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands. Emailchris.lee@arstechnica.com//Twitter@exMamaku

There is no "arrow of time dilemma" either for physicists or philosophers. That's the first issue I have with the article.

The second issue is that "the laws of physics" include one directional laws - in particular statisical x (where x is thermodynamics, mechanics etc etc). In these areas time is a measure of change (at a first approximation - an index) and increases 'a priori'

So the loveable Ph.D. has answered two questions that already had answers, and seems to be claiming invention of the wheel a little too late. ie no new invention required - already has a "device that goes round on an axle". Thank you very much.

Originally posted by Raineer:So the idea is basically that it's a lot more difficult to turn this pile of ashes into a log of wood than it is to turn a log of wood into ashes....

Add to that how, if you observed the log burning to ash, to reverse the state you would have to forget that it ever did so. If you forgot that it happened, you have effectively not observed it taking place and so it might as well have never happened. There's no way to know if such entropy-reversals take place, therefore, because we wouldn't be able to observe them to know that something has changed. They may as well not take place. That's what I get from this explanation of the paper.

Originally posted by Wheels Of Confusion:Add to that how, if you observed the log burning to ash, to reverse the state you would have to forget that it ever did so. If you forgot that it happened, you have effectively not observed it taking place and so it might as well have never happened.

As an extension to this thought experiment, let us suppose that he is correct and I have a time machine. If I move forwards in time (faster than I normally would), then I can bring information with me. But if I move backwards in time, then I cannot take any information with me, including me and the time machine. Now that I've devised a relatively simple test to his hypothesis, who's willing to loan me their time machine?

No, I don't think "sokaled" should be a verb. The whole "Sokal affair" as it has come to be called was an embarrassing set of actions by grown people who really should have known better. The Social Text editors should not have been operating a journal without the safety net of peer review, and there was no real point in Sokal using the silly extremes of a group of practitioners to debunk a larger set of ideas which had actually produced some interesting results and managed to lift American literary criticism out of the abyss of either strictly adhering to intentionality or claiming texts existed in some sort of ethereal sphere away from messy meat-based forms of life. The Sokal affair In a nutshell: scientist debunks humanists debunking science. (Or, rather, scientist gets to be a bad boy calling attention to other bad boys.) Yeah, that's the kind of antics we want to perform for an American public that otherwise doesn't believe the last thirty years of science ever happened and that humanists are the anti-Christ in collective form.

Let's leave that asininity behind and get on with the better kinds of dialogues that seem to be taking place lately.

Originally posted by laudunum:No, I don't think "sokaled" should be a verb. The whole "Sokal affair" as it has come to be called was an embarrassing set of actions by grown people who really should have known better. The Social Text editors should not have been operating a journal without the safety net of peer review, and there was no real point in Sokal using the silly extremes of a group of practitioners to debunk a larger set of ideas which had actually produced some interesting results and managed to lift American literary criticism out of the abyss of either strictly adhering to intentionality or claiming texts existed in some sort of ethereal sphere away from messy meat-based forms of life. The Sokal affair In a nutshell: scientist debunks humanists debunking science. (Or, rather, scientist gets to be a bad boy calling attention to other bad boys.) Yeah, that's the kind of antics we want to perform for an American public that otherwise doesn't believe the last thirty years of science ever happened and that humanists are the anti-Christ in collective form.

Let's leave that asininity behind and get on with the better kinds of dialogues that seem to be taking place lately.

Actually, Sokal was intent on making a point, and that he did (IMHO). He wanted to prove that "social sciences" publications would publish a load of rubbish if it appeared to be in line with current fashion in that area. And he got his paper published.

Similarly, there is a lot of debate on the reliability of scientific publications, or reference to scientific research, especially in such complex times when there is pressure to publish not only from academia but also from financers. Lots of good people spend time trying to instill some degree of rationality in the audiences - check out www.badscience.net or http://realclimate.org for some examples.

While scientists may be able to distinguish trash from valid publications, they're often less likely to waste time disproving what to them is obviously nonsense. In the meantime, the general public may get a distorted coverage of those issues because they do not have the training or even common sense to spot a shady author.

That said, it's kind of distasteful to compare the author of the work mentioned in this piece to Alain Sokal. Either Chris Lee thinks that Maccone's paper is a fraud, or he doesn't. No sense in just dropping the bomb and running for cover.

Originally posted by Wheels Of Confusion:Add to that how, if you observed the log burning to ash, to reverse the state you would have to forget that it ever did so. If you forgot that it happened, you have effectively not observed it taking place and so it might as well have never happened. There's no way to know if such entropy-reversals take place, therefore, because we wouldn't be able to observe them to know that something has changed. They may as well not take place. That's what I get from this explanation of the paper.

This as well as that every single thing in the universe would have to reverse mid-action instantaneously. Can you imagine the momentum change? Momentum would still be conserved, but where would the change in action come from? Two particles in a collision is easy to get, but what about two particles moving directly away from each other stopping in an instant (nothing like potential energy problems you do in classical mechanics) and take a collision course. It's unintuitive as all hell. On what level of reality could the action said to occur and be feasibly explained?Pac-Man?

Originally posted by laudunum:No, I don't think "sokaled" should be a verb. The whole "Sokal affair" as it has come to be called was an embarrassing set of actions by grown people who really should have known better.

Originally posted by laudunum:That said, it's kind of distasteful to compare the author of the work mentioned in this piece to Alain Sokal. Either Chris Lee thinks that Maccone's paper is a fraud, or he doesn't. No sense in just dropping the bomb and running for cover.

While I would usually agree, this is suppose to be news. I am not interested in the reporter's opinion.(No Offense, Chris Lee) But since the idea "Sokaled" is relevant to the paper in an abstract way, I think it was entirely appropriate for him to pose the question and let us decide for ourselves.

I read the paper fairly carefully, here's my summary and take. (I'm a PhD candidate in physics, but quantum information is not my area of expertise.)

First of all, the paper examines the nature of entropy decreasing events rather than time reversal as such. Second, it is developed using a quantum informational (non-classical) definition of entropy. Third, it examines the relationship between the entropy of a system and the correlations between various subsystems. These correlations allow one, in principle, to draw conclusions about one subsystem by measurements on a different one. A main assumption here is that all information is physical (so that the researcher's brain and lab notebook are not excluded), and that any information which can be gained about a system indirectly *must* be obtained through the use of such correlations. This last point is repeated several times: uncorrelated systems necessarily contain no information about each other.

The main mathematical result is that, under the assumption of a closed system which is and remains a pure state, the sum of changes in entropy of the various subsystems minus the changes in mutual quantum information between various subsystems (i.e. the correlations) sum to 0. A pure state has 0 entropy, and a pure state undergoing unitary transformations (which are reversible by definition) remains pure. This is a critical assumption, and although Maccone doesn't state it explicitly I presume it is justified because quantum states can be purified. In other words, for an arbitrary system A there exists (mathematically speaking) system B such that A is a subsystem of B and B is pure.

The upshot of the mathematical result is that if some subsystem increases in entropy and then decreases again, the correlations which let another subsystem (whether a conscious observer or anything else) see the increased entropy in the first place disappear when the entropy decreases again. Consider a system that has entropy S0, increases to S1, and then decreases to S2, where S1>S2>S0. The overall entropy in that system has increased with time, but the increases from (S2-S0) to S1 and then back to S2 are necessarily uncorrelated with the observer system, and so no information exists that such an increase and decrease ever took place, and the only remaining evidence is of the increase from S0 to S2. (He also states that "the memory of an event...has nonzero classical mutual information" about that event, and goes on to relate classical mutual information to quantum mutual information, but it's unclear to me whether or not this premise is correct or necessary to make the point).

The author notes that, from the perspective of undoing these correlations, it isn't necessarily the case that the system's dynamics underwent an exact reversal, and so his conclusion is even broader than the time's arrow question alone. (I would like to see an example of this non-trivial "erasure" without using exact reversal...) Nevertheless, it is necessarily true that an exact time reversal of any system will decrease that system's entropy if the "forward-time" version increased it. But then, by the arguments just outlined, all correlations which recorded this decrease in entropy are destroyed and without those correlations no system can be aware of the time reversal.

This was a very interesting read, and I'll be interested to see how the physics community responds to it. I personally found it rather persuasive: it would explain (at least in part) why events we would expect to observe in a world of unitary transformations are not observed, from within the perspective of the science which predicts these unitary transformations in the first place. That's not the same as explaining why time has an arrow to begin with (the anthropic principle suffers a similar problem as an "explanation") but it does address a seeming inconsistency.

Also, I didn't get any "Sokal-like" alarms from the jargon, but I admit I am ignorant about several of the tools utilized.

Originally posted by Corporate_Goon:Richard Feynman covered pretty much exactly this in a lecture on computers and memory about 25 years ago.

I remember something about this.

Feynman was talking about computer memory and the amount of power it takes to undo operations. If you didn't keep a record then you could undo a write to memory and consume a net of no energy. If you wanted to keep a record then that consumed power. The more complex the record the more power needed.

I also remember Daniel Deutsch had some equivalence principle that could be crudely stated as: the universe is a computer. Add this to Feynman's work and you got Maccone's work. (I leave it to you guys to fill in the steps.)

But I have to admit that I love Feynman's work on reversibility in computers as he created a set of computer logic gates based on elastical colliding balls (this was to stop confusion over quantum effects). I spent hours tracing out the movement of little balls.

Originally posted by laudunum:That said, it's kind of distasteful to compare the author of the work mentioned in this piece to Alain Sokal. Either Chris Lee thinks that Maccone's paper is a fraud, or he doesn't. No sense in just dropping the bomb and running for cover.

While I would usually agree, this is suppose to be news. I am not interested in the reporter's opinion.(No Offense, Chris Lee) But since the idea "Sokaled" is relevant to the paper in an abstract way, I think it was entirely appropriate for him to pose the question and let us decide for ourselves.

Edit:-I think this was a stab at humor, I laughed.

This comment was edited by SgtCupCake on August 19, 2009 01:06

This may come as surprise to some, but I am a working physicist and a reporter. I read these papers with the eyes of a scientist and, more importantly, I know my limitations. For instance, I know that if someone really wanted to, they could indeed sokal me. How do I know this is true? Google Bell labs and Jon Hendrik Schön. If I had been convinced it was a sokal, I wouldn't have written about it. You will notice that I was suspicious enough to check out the authors publication record--something I almost never do. But, unlike Laudunum, I don't think I should hide those suspicions from my audience.

As for the arrow of time being a solved problem: no, it isn't. Many physicists spend a good deal of time discussing the arrow of time, and there have been many approaches to trying to solve it, but there is no consensus yet over how we get an arrow. You can, in fact, go further and state this problem as "we have trouble defining the arrow of time because we don't know what time is." If you prefer an argument from authority, the evidence is right before you. I know that if I sent in something to PRL about say: "solving the problem of gravity" or showing that "light amplification by stimulated emission" was possible, I would be rejected out of hand. PRL doesn't make a habit of publishing solved problems.

Originally posted by Ainamacar:The upshot of the mathematical result is that if some subsystem increases in entropy and then decreases again, the correlations which let another subsystem (whether a conscious observer or anything else) see the increased entropy in the first place disappear when the entropy decreases again.

I'm not in the biz at all, so you can ignore this if this question is too ignorant: but since we're talking about the entropy of a subsystem, not the overall closed system of which the observer is a part, couldn't the entropy reversal (S1 to S2) come not through a simple 'undo' on the part of the larger system (that is, undoing whatever process delivered the information from the observed subsystem to the observer) but instead through some further operation by a third subsystem (perahps not even observed by the original observer)? Furthermore, if S0 to S1 produces record R1 in the observer at the cost of some entropy delta less than S1-S0, couldn't an operation which lowered the subsystem to S2 produce a second record R2, provided only that the operation also compensated for whatever entropy in the observed subsystem the recording process incurred?

In other words, it's hard to see how this holds, or is even interesting, when you operate at the level of subsystems. (Of course, the quantum-physics definition of all these terms (information, entropy, etc) may be too unrelated to their normal meanings for any of this reasoning to apply, I guess.)

How does this resolve the arrow of time problem? Well, put simply, running time in one direction allows records to be kept and events to be observed. In the other direction, observation becomes impossible. Therefore, although time could be running in either direction (or, who knows, both directions simultaneously), it is only possible for any observer (not necessarily a human one) to experience time in the forward direction..

If by records and observation you mean effects on environment, I would argue the Universe records itself. I know I have made this argument before but, if we, humanity as a whole, dosen't observe a star does it still not burn? Some other matter in the universe is affected by the star burning and in it's way "observes" it. This nails the star to our reality with times arrow moving in one direction.If you wanted to run time backward, you would have to run the entire universe in reverse because we are all in the river of time together and affect each other. I mean we the energy and matter of the universe.-Carrieedit: Ah, it seems Arrowsmith beat me. Also, I think Ainamacar although I'm not able to follow his argument, kinda goes over my head.

posted by laserboy in the comments:As for the arrow of time being a solved problem: no, it isn't. Many physicists spend a good deal of time discussing the arrow of time, and there have been many approaches to trying to solve it, but there is no consensus yet over how we get an arrow. You can, in fact, go further and state this problem as "we have trouble defining the arrow of time because we don't know what time is." If you prefer an argument from authority, the evidence is right before you. I know that if I sent in something to PRL about say: "solving the problem of gravity" or showing that "light amplification by stimulated emission" was possible, I would be rejected out of hand. PRL doesn't make a habit of publishing solved .

Perhaps the arrow of time problem is the result of trying to use mathmatics to describe physical, real world systems. Much like Euclidean geometry is good for most real world work but is not real. A point or 2 dimensional line do not really exist, they are thought tools we use.-Carrie

First of all,it is said that perhaps 90+ percent of the material in the universe is not defined. Some of Einsteens ideas- the relativity of an observer (such as that of a person onboard two equivalent speeding trains),and that of E=MC2. Well even in chemistry,there are materials that are simply utilizing existance (as in atoms) that are existant to its traits of similiar observers. This makes the quantifyable rational of their "time"among them. One material is used to quantify,and qualify its ''time''. The metric of its existance to the rest of the universe. With E=MC2,actually 'time'is not represented. It will be true to several distinc atoms,but not quantifyable,or not rational equavalents to others. "Space"is also not realized with this. So both 'time',and 'space'are somehow missed in the metric of the observer.

______________________ I have often wondered in the event of the phrase 'energy can be neither created or destroyed'. Since it is (to us as observers) dissapearing. As it is used. Certainly the measurement of it does not change its potential or use - e.g. horsepower,watts etc. Then,what about the ideal of reversing the electrical connections of a DC engine ? If you switch the connections the engine runs ....backward ?

The potential in my world,is that the relationship is that of a connected relativity between the 'flow'of the materials conducting the electrons. The material 'time'are in corresponding equivalents. But the ideal is the observer of the 'work'(the engine going backward).

Most electric wizards will simply state that the 'potential',has simply changed. But I question that in fact,the very existance of the atomic space is the relationship of a balance of both abstracts of 'forward,and backward time states'. Components of which the atoms actually are.

Of course this would be considered bizzare. And probably be thought of at some earlier time,by a different person.

As far as entropy is concerned,I'm under the defines that 'things are falling apart faster than can be put together',and that 'for every action there is an opposite and equal reaction'. So that Beurnolis principle added to these makes for creative precedence.

The equations are there but the metrics are not exact for the observations. I think that there is a 'backward time in the universe,and a forward time in the universe,and we live in the balance between them. Where 'energy is neither created or destroyed',we simply are not supplied enough in the balance to be observers . Or the metrics are not resolved. Every know ideal is a leverage between what is before,and what is after.